Principles of Economics is a leading political economy or economics textbook of Alfred Marshall (1842–1924), first published in 1890. It was the standard text for generations of economics students. Called his magnum opus, it ran to eight editions by 1920. A ninth (variorum) edition was published in 1961, edited in 2 volumes by C. W. Guillebaud.

Political economy is a branch of political science and economics studying economic systems and their governance by political systems. Widely studied phenomena within the discipline are systems such as labour markets and financial markets, as well as phenomena such as growth, distribution, inequality, and trade, and how these are shaped by institutions, laws, and government policy. Originating in the 16th century, it is the precursor to the modern discipline of economics. Political economy in its modern form is considered an interdisciplinary field, drawing on theory from both political science and modern economics.